BU-910: How to Repair a Battery Pack

Batteries for power tools and other commercial devices can often be repaired by replacing one or all cells. Finding a NiCd and NiMH cell is relatively easy; locating an appropriate Li-ion cell is more difficult. Quality Li-ion cells are not readily available because a reputable battery manufacturer only sells to certified pack assemblers(See BU-305: Building a Lithium-ion Pack) In addition, quality Li-ion cells may only be available in high quantity, leaving smaller service enterprises in a disadvantage.

When repairing a Li-ion pack, make certain that each cell is properly reconnected to a protection circuit(See BU-304: Why are Protection Circuits Needed? and BU-304a: Safety Concerns with Li-ion) With the exception of some single-cell and power-tool packs, all Li-ion batteries must have a protection circuit; lead and nickel-based batteries are exempt.

Lead- and nickel-based batteries are more or less uniform and one can say that lead acid is lead acid and NiMH is NiMH. This cannot be uttered about the lithium-ion family as it has diverged into many unique systems and some with different voltages. While most Li-ion has a nominal cell voltage of 3.60V and charges to 4.20V, some specialty Li-ion charge to 4.10V and newer Energy Cells top at 4.35V/cell and higher. Li-phosphate presents a further exception with a nominal cell voltage of 3.20V and a charge limit of 3.65V/cell. Also unique is Li-titanate with a nominal cell voltage of 2.40V and charge limit of 2.85V(See BU-205: Types of Lithium-ion) These cells are definitely not interchangeable.

Besides different chemistries, cobalt-based Li-ion is divided into two camps: The Energy Cell provides maximum runtime at moderate load currents and regular charge time; the Power Cell offers high power and some permit shorter charge times(See BU-501: Basics About Discharging) When replacing the cells of a power tool battery, use Power Cells of identical specification to give the pack the same strength and endurance as the previous cell set. Communications devices, cameras and e-bike use the Energy Cell.

Do not go cheap on the cells and source them from an unrecognized manufacturer. Failing packs often trace to inferior cells, and this also applies with the popular 18650 Li-ion format that come in good and bad qualities. One hears repeatedly of companies that took advantage of marked-down prices from a clearing house, only to have the packs fail after 2–3 years because of high cell failure.

If a relatively new pack has only one defective cell and a replacement is located, exchanging the affected cell makes sense. With an aged battery, however, it’s best to replace all cells. Mixing new with old causes a cell mismatch that has a short life. In a well-matched battery pack all cells have similar capacities. An anomaly is a chain in which the weakest link determines the performance of the battery(See BU-302: Serial and Parallel Battery Configurations)

Cells designed for a multi-cell pack require tighter tolerances than those destined for single-cell use, such as a mobile phone. Cell manufacturers cannot fully control the capacity and hence some have higher capacities than others. In a single-cell device the average consumer does not notice such a variance. Quality cells for a multi-cell pack, on the other hand, are capacity-matched, lest they fail.

A battery shop may salvage good cells from a failed pack for reuse but the recovered cell should be checked for capacity, internal resistance and self-discharge – the three key health indicators of a battery. When checking a cell with a battery analyzer, mark the capacity so it can be matched with a pack that may need a cell of similar capacity level. Also make certain that the internal resistance is in par with a good cell and verify the self-discharge. Electro-mechanical stress often manifests itself in elevated self-discharge. To check self-discharge, fully charge the cell, measure the voltage and compare the voltage after 24 hours and perhaps a week with the voltage drop of known good cells.

Visitors of BatteryUniversity.com frequently ask: “Can NiCd can be replaced with NiMH?” In theory, this should be possible as both chemistries have the same cell voltage, but full-charge detection and trickle charge are the issues. NiMH uses a more refined charge algorithm than NiCd(See BU-408: Charging Nickel-metal-hydride) A modern NiMH charger can charge both NiMH and NiCd; the old NiCd charger could overcharge NiMH by not properly detecting the full-charge state and applying a trickle charge that is too high. Each chemistry requires its own charger.

Spot-welding a cell is the only reliable way to get dependable connection and four spot-welds per cell is the norm. Limit the heat transfer to the cells during welding to prevent damage. Insulate each cell electrically as the cell housing is “hot” and carries a voltage. If the new cell is at different charge level than the existing ones, apply a slow charge to bring them all to the same level. Observe the temperature during charge. Nickel-based cells will warm up towards the end of charge but must cool down on ready; Li-ion should stay cool during charge. The rise in temperature should be equal for all cells; unevenness hints to an anomaly.

Measure the voltage of a repaired pack and check it again after 24 hours and a few days. If a cell drops lower than another in the pack, then there is fear of elevated self-discharge(See BU-802b: Elevated Self-discharge)

Simple Guidelines when Repairing Battery Packs

• Only connect cells that are matched. Do not mix cells of different chemistry, age or capacity.

• Never charge or discharge Li-ion batteries unattended without a working protection circuit. Each cell must be monitored individually with a protection circuit.

• Include a temperature sensor that disrupts the current should the pack get hot.

• Apply a slow charge to a repaired pack to bring all cells to parity.

• Pay attention when using an unknown cell brand. Elevated temperature hints to an anomaly.

• Do not charge a Li-ion battery that has physical damage, has bulged or has dwelled at a voltage of less than 1.5V/cell for some time.

• Check a repaired pack for self-discharge. Intrinsic defects often reveal themselves in an elevated self-discharge.